Rizevim has great trust in Euclid as he considers him to be his right hand man. In exchange as a Lucifuge, Euclid Is highly loyal to Rizevim as the son of the original Lucifer, he respectfully calls him Rizevim-sama. Euclid's loyalty to Rizevim is so great that Euclid willingly assist Rizevim in fufilling the latter's atrocious ambitions despite having absolutely no personal benefits in doing so, going as far as to adopt Rizevim's ideology that a Devil must be evil as his own claiming that Rizevim's ideas are necessary for him.
His older sister who he admired when he was younger for Grayfia's courage and her strength, however that admiration was shattered when she rebelled against the original Lucifer and fell in love with Sirzechs. Grayfia's betrayal towards Lucifer caused Euclid to fell into despair and was left with emotional scars that never healed for centuries. Despite baring a grudge against Grayfia for going against the original Lucifer, Euclid appears to still have great love and attachment for her, going as far as to kidnap Rossweisse due to her slight resemblance to his sister.
Euclid has a personal grudge and hatred against Sirzechs for having his sister Grayfia's love which is what lead her to betray Lucifer as well as usurping the original Lucifer's position as the ruler of the Underworld, even so he still refers to him as "brother" albeit in a sarcastic manner.
Main Characters Edit
Issei and Euclid's relationship started immediately hostile, as the latter was responsible for the abduction of the first year members of the Occult Research Club and sending several Magicians to attack them and even aiding the revival of the Evil Dragons.
Furthermore, later on, both Issei and Ddraig developed a personal grudge against Euclid for creating replica of the Boosted Gear from the remains of Issei's previous body left in the Dimensional Gap and sacrificing the souls of many famous Dragons.
During their fight in Volume 16, Euclid was able to overpower Issei's Cardinal Crimson Promotion with his own Scale Mail armor in it's normal Balance Breaker form, this states that Euclid's natural strength is equivalent to a Satan's even without the armor. However Issei was able to turn the tables on Euclid only after using his newly awakened power and further theories that Euclid was observing him in order to learn how to use more of his new techniques.
Euclid took an interest in Rossweisse in Volume 17, for her magical abilities and most of all on how she resembles his sister Grayfia, going as far as to kidnapping her which ultimately failed.